The present invention relates to an emitter-coupled multivibrator and, more particularly, to an emitter-coupled multivibrator including two transistors which are alternately turned on and off and the emitters of which are connected with each other via a capacitor and are also connected to respective constant current sources.
In a field of directly modulating the frequency of a baseband signal such as a video signal which requires a wide frequency deviation, a frequency modulator is usually constituted by a circuit based on an emitter-coupled multivibrator, which is one of the relaxation oscillation system. FIG. 6 shows an example of the circuit of such an emitter-coupled multivibrator, and FIG. 7 shows the waveform of the operation of each element thereof.
In FIG. 6, the symbols Q.sub.1, Q.sub.2 represent NPN transistors which constitute a multivibrator and which are alternately turned on and off, C a capacitor for connecting the emitters of the transistors Q.sub.1, Q.sub.2, and CS.sub.1, CS.sub.2 constant current sources provided between the emitters of the respective transistors Q.sub.1, Q.sub.2 and the earth. A frequency controlling voltage V.sub.C is input to the each of the constant current sources CS.sub.1, CS.sub.2, which supply a constant current I.sub.C represented by the following equation: EQU I.sub.C =gm.multidot.V.sub.C
wherein gm represents a mutual conductance. The collectors of the transistors Q.sub.1 and Q.sub.2 are connected to a bias supply V.sub.CC via resistors R.sub.1, R.sub.2 and diodes D.sub.1, D.sub.2, respectively. The diode D.sub.1 (D.sub.2) has a function of clamping the terminal voltage of the resistor R.sub.1 (R.sub.2) to a constant voltage V.sub.D when the transistor Q.sub.1 (Q.sub.2) is turned on even if a collector current changes.
The symbols Q.sub.3, Q.sub.4 represent NPN transistors for supplying a base current to the transistors Q.sub.1 and Q.sub.2. The emitter terminals of the transistors Q.sub.3 and Q.sub.4 are connected to the base terminals of the transistors Q.sub.2 and Q.sub.1, respectively, the collector terminals are connected to the bias supply V.sub.CC, and the base terminals are connected to the collector terminal of the transistors Q.sub.1 and Q.sub.2, respectively.
If it is now assumed that the transistor Q.sub.1 is on and the transistor Q.sub.2 is off, as shown in FIG. 6(a), the emitter potential of the transistor Q.sub.1 is V.sub.CC -2V.sub.BE, wherein V.sub.BE is a voltage between the base and the emitter. Although the emitter potential of the transistor Q.sub.2 is inferred to be V.sub.CC -V.sub.D -2V.sub.BE, the inter-base emitter voltage of the transistor Q.sub.2, actually, cannot reach V.sub.BE which is necessary for turning on, because the emitter-coupled capacitor C is charged as shown in FIG. 6(a). Therefore, the transistor Q.sub.2 is still off. In this state, if the constant current I.sub.C flows in the direction indicated by the arrow via the transistor Q.sub.1 which is on, the charges stored in the coupling capacitor C are discharged, and the capacitor C is finally charged to the opposite polarity. If the charging state continues, the emitter potential of the transistor Q.sub.2 is lowered and the inter-base emitter voltage has a satisfactory V.sub.BE which is sufficient for turning on the transistor Q.sub.2, and a current begins to flow through the transistor Q.sub.2, which is turned on.
When the transistor Q.sub.2 is turned on, as shown in FIG. 6(b), a diode clipping voltage V.sub.D generates as a terminal voltage of the resistor R.sub.2 which is connected to the collector terminal of the transistor Q.sub.1 resulting in lowering the base potential of the transistor Q.sub.1. But, the emitter potential of the transistor Q.sub.1 cannot be lowered in accordance with the lowered base potential, because the capacitor C is charged in the polarity shown in FIG. 6(b), so that the transistor Q.sub.1 is turned off. This is the operation in a half cycle.
The peak-to-peak amplitude of the rectangular wave of the collector voltage of each of the transistors Q.sub.1 and Q.sub.2 is the clamping voltage V.sub.D(p-p) of the diodes D.sub.1 and D.sub.2 respectively, and the emitter potential has an intermittent saw-tooth waveform. The amplitude thereof is 2V.sub.D(p-p) with the charging voltage taken into consideration. Consequently, the time (T/2) elapsed before the emitter potential of the transistor Q.sub.1 or Q.sub.2 drops by 2V.sub.D is represented by the following equation: EQU (1/C).multidot..intg.I.sub.C dt=2V.sub.D EQU t=T/2=2V.sub.D C/I.sub.C =2V.sub.D C/gm.multidot.V.sub.C (1)
Therefore, the oscillation frequency f.sub.0 is represented by EQU f.sub.0 =1/T=(gm/4CV.sub.D).multidot.V.sub.C (2)
In an actual diode, however, as is clear from the voltage-current characteristic of a diode, the clamping voltage (V.sub.D) of the diode fluctuates slightly depending upon the current I.sub.C, i.e., the frequency controlling voltage V.sub.C. Consequently, the oscillation frequency f.sub.0 is sometimes not proportional to V.sub.C.
To solve this problem, a circuit has been proposed which is improved in that the amplitudes of the intermittent saw-tooth waves (amplitudes of the waveforms of the emitter potentials of the transistors Q.sub.1 and Q.sub.2) do not directly depend upon the clamping voltages VD of the diodes D.sub.1 and D.sub.2.
FIG. 8 shows the circuit of such an improved emitter-coupled multivibrator, and FIG. 9 shows the waveform of the operation of each element thereof. In FIG. 8, the same reference numerals are provided for the elements which are the same as those shown in FIG. 6. The emitter-coupled multivibrator shown in FIG. 8 is different from that shown in FIG. 6 in that a current switch CSW is provided. The current switch CSW is composed of a differential pair of amplifiers of the transistors Q.sub.3, Q.sub.4, a current source CS of a constant current I, and collector resistors R.sub.C, etc.
When the transistor Q.sub.1 is on, and the transistor Q.sub.2 is off, the transistor Q.sub.3 in the current switch SW is off and the transistor Q.sub.4 is on. In this case, a limit voltage V.sub.LMT (=I.multidot.R.sub.C) having a constant amplitude generates at the collector terminal of the transistor Q.sub.3, and the limit voltage V.sub.LMT is input to the base terminal of the transistor Q.sub.1 via the coupling capacitor. On the other hand, the voltage at the collector terminal of the transistor Q.sub.4 is 0, and this voltage is input to the base terminal of the transistor Q.sub.2.
In this state, a constant current I flows through the emitter coupling capacitor C in the direction indicated by the arrow so as to discharge the charges stored in the coupling capacitor C and then charge the capacitor C to the opposite polarity. If the charging state continues, the emitter potential of the transistor Q.sub.2 is lowered and the inter-base emitter voltage has a satisfactory V.sub.BE which is sufficient for turning on the transistor Q.sub.2, and a current begins to flow through the transistor Q.sub.2, which is turned on. Then the base potential of the transistor Q.sub.3 which constitutes the differential pair becomes higher than the base potential of the transistor Q.sub.4, and the transistor Q.sub.3 is turned on, while the transistor Q.sub.4 is turned off. When the transistor Q.sub.3 is turned on, the base potential of the transistor Q.sub.1 is lowered to approximately zero. However, the emitter potential of the transistor Q.sub.1 cannot be lowered in accordance with the lowered base potential, because the capacitor C is in the act of charging, so that the transistor Q.sub.1 is turned off. This is the operation in a half cycle.
As described above, the transistor Q.sub.1 is turned off and the transistor Q.sub.2 is turned on, and the transistor Q.sub.3 is turned on and the transistor Q.sub.4 is turned off in the current switch CSW in the first half cycle. In addition, the limit voltage V.sub.LMT having a constant amplitude (=I.multidot.R.sub.C) generates at the collector terminal of the transistor Q.sub.4, and the limit voltage V.sub.LMT is input to the base terminal of the transistor Q.sub.2 via the coupling capacitor. On the other hand, the voltage at the collector terminal of the transistor Q.sub.3 is 0, and this voltage is input to the base terminal of the transistor Q.sub.1. In this state, the same operation as that in the first half cycle is repeated in the second half cycle.
As is clear from the above explanation, the peak-to-peak amplitude of the rectangular wave of the collector voltage of each of the transistors Q.sub.1 and Q.sub.2 is the clamping voltage V.sub.D(p-p) of the diode D.sub.1 (D.sub.2), but the amplitude of the rectangular wave of the base of each of the transistors Q.sub.1 and Q.sub.2 is V.sub.LMT. That is, the V.sub.C dependence of the amplitude of the rectangular wave of the base is removed by the limiter characteristic of the current switch CSW, and the amplitude becomes V.sub.LMT. As a result, the emitter potential of each of the transistors Q.sub.1 and Q.sub.2 has an intermittent saw-tooth waveform and the amplitude thereof is 2V.sub.LMT with the charging voltage taken into consideration. Consequently, the time (T/2) elapsed before the emitter potential of the transistor Q.sub.1 or Q.sub.2 drops by 2V.sub.LMT is represented by the following equation: EQU T/2=2V.sub.LMT C/I.sub.C =2V.sub.LMT C/gm.multidot.V.sub.C (3)
Therefore, the oscillation frequency f.sub.0 is represented by EQU f.sub.0 =1/T=(gm/4CV.sub.LMT).multidot.V.sub.C (4)
In an emitter-coupled multivibrator improved in the above-described way, the V.sub.C dependence of the amplitude of the rectangular wave of the base is removed and, as a result, it is possible to make the oscillation frequency f.sub.0 proportional to the frequency controlling voltage V.sub.C. However, even in such an improved emitter-coupled multivibrator, the Vc-f.sub.0 characteristic becomes nonlinear with an increase in the oscillation frequency f.sub.0, as shown in FIG. 10. This is because the rising time in the intermittent saw-tooth waveform cannot be regarded as zero. More specifically, in a case where a high frequency oscillation generates, it is impossible to disregard the rising time in the intermittent saw-tooth waveform, as shown in FIG. 11. If it is assumed that the rising time is constantly .tau., the half period takes longer by the time corresponding to the rising time .tau.. The relational expression for obtaining the half period is therefore corrected as follows: EQU T'/2=2(V.sub.LMT C/gm.multidot.V.sub.C)+.tau. (5)
Therefore, the oscillation frequency f.sub.0 is represented by EQU f.sub.0 =(1/T')=gm.multidot.V.sub.C /(4CV.sub.LMT +2.tau..multidot.gm.multidot.V.sub.C) (6)
At this time, the V.sub.C dependence generates in the denominator, and the V.sub.C-f.sub.0 characteristic has a nonlinear characteristic as shown in FIG. 10.